1. Field of the Invention
The present invention relates to a light emitting device assembly, and more particularly, to a flip-chip bonding type light emitting device assembly.
2. Description of the Related Art
Generally, a laser beam of a laser diode (LD), which has a narrow frequency bandwidth and a good property of going straight ahead, has been recently put to practical use in the fields of optical communications, multiple communications, and space communications. One of major applications of the LD, along with the optical communications, is a pickup for an optical disc.
A GaN semiconductor LD includes a plurality of compound semiconductor layers, which are grown by crystalline grain growth and stacked on a substrate formed of sapphire, GaN, or SiC. Specifically, such compound semiconductor layers as an n-type GaN contact layer, an n-type AlGaN/GaN clad layer, an n-type GaN wave guide layer, an InGaN active layer, a p-type GaN wave guide layer, a p-type AlGaN/GaN clad layer, and a p-type GaN contact layer are sequentially stacked on the substrate. In the GaN semiconductor LD having the above-described stack structure, a ridge for forming a p-type electrode is provided on the uppermost portion of the LD. A passivation layer is formed between the ridge and the p-type electrode to define a conducting path in the semiconductor stack structure.
The GaN semiconductor LD is flip-chip bonded to a submount to facilitate transfer of heat generated during drive. By the flip-chip bonding, a top surface of a light emitting device, such as the LD where the p-type electrode is formed, is fixed to the submount using a solder bump.
When the light emitting device is flip-chip bonded to the submount, since the light emitting device and the submount are directly connected to each other using the solder bump, thermal resistance and line resistance are reduced as compared with when wire bonding is used.
FIG. 1 is a schematic cross-sectional view of a light emitting device assembly in which a light emitting device is flip-chip bonded to a submount.
In FIG. 1, reference numeral 10 denotes the light emitting device, such as an LD, and 21 denotes the submount. The light emitting device 10 is turned over to be bonded to the submount 21. The light emitting device 10 includes a compound semiconductor layer 12 and a substrate 11 on which the compound semiconductor layer 12 is grown. The compound semiconductor layer 12 includes an n-type compound semiconductor layer (not shown), a p-type compound semiconductor layer (not shown), and an active layer (not shown) disposed therebetween. A first pad layer 22a and a second pad layer 22b are formed on the submount 21. The first pad layer 22a and the second pad layer 22b respectively face two stepped regions of the compound semiconductor layer 12 and are spaced apart from each other. The two stepped regions of the compound semiconductor layer 12 correspond to a region where an n-type electrode (not shown) is formed and a region where a p-type electrode (not shown) is formed, respectively. There is a step difference S between the two stepped regions. A pad 13a is formed in the region where the n-type electrode is formed, and a pad 13b is formed in the region where the p-type electrode is formed. The pads 13a and 13b are in contact with the n-type electrode and the p-type electrode, respectively.
A solder bump 30a is interposed between the pad 13a disposed on the compound semiconductor layer 12 and the corresponding first pad layer 22a disposed on the submount 21, and a solder bump 30b is interposed between the pad 13b disposed on the compound semiconductor layer 12 and the corresponding second pad layer 22b disposed on the submount 21. The solder bumps 30a and 30b each include stacked conductive materials.
Specifically, the solder bump 30a includes a first gold layer 31a that contacts the pad 13a of the light emitting device 10, a first platinum layer 33a that contacts the first pad layer 22a, and an AuSn solder 32a disposed therebetween. Likewise, the solder bump 30b includes a second gold layer 31b that contacts the pad 13b of the light emitting device 10, a second platinum layer 33b that contacts the second pad layer 22b, and an AuSn solder 32b disposed therebetween.
The light emitting device 10 is turned over and the solder bumps 30a and 30b are closely bonded to the pads 13a and 13b, respectively, due to a predetermined pressure. In this state, the resultant structure is heated for several seconds to a temperature of about 280° C. or higher. As a result, the light emitting device 10 is fixed to the submount 21 by the solder bumps 30a and 30b. However, in this conventional light emitting device 10, the pads 13a and 13b are unreliably bonded to the solder bumps 30a and 30b, respectively, because of the following reasons.
Typically, the pads 13a and 13b of the light emitting device 10 are patterned using lift-off. When lift-off is used, as shown in a left view of FIG. 2, which illustrates a conventional light emitting device assembly before bonding, a flare type fence 14 is formed along an edge of each of the pads 13a and 13b. Accordingly, as shown in a right view of FIG. 2, which illustrates the conventional light emitting device assembly after bonding, when the pads 13a and 13b come close to the bumps 30a and 30b, the pads 13a and 13b become the first portions that contact the top surfaces of the bumps 30a and 30b. After the pads 13a and 13b are closely bonded to the bumps 30a and 30b, a gap is formed between the pads 13a and 13b and the bumps 30a and 30b due to the protruding fences 4.
FIG. 3A is a microscopic photograph of the pads 13a and 13b of the light emitting device 10, FIG. 3B is an exploded scanning electronic microscope (SEM) photograph of a portion illustrated with a left circle shown in FIG. 3A, and FIG. 3C is an exploded SEM photograph of a portion illustrated with a right circle shown in FIG. 3A.
As shown in FIG. 3A, pads are disposed on both outer portions of the light emitting device 10. The pads are formed using a typical method, i.e., lift-off. Thus, as described above, fences are formed along edges of the pads, respectively. It can be seen from FIGS. 3B and 3C that metal fences are formed during the lift-off. The metal fences are produced when a metal material is torn off during the lift-off, and each of them has a greater height than adjacent portions.
FIG. 4A is a photograph of the conventional light emitting device assembly in which the light emitting device is fixed to the submount, and FIG. 4B is a photograph of a plan view of a solder bump of the submount after the light emitting device is separated from the submount. In the light emitting device assembly bonded as shown in FIG. 4A, a gap occurs between the solder bump and the pad of the light emitting device and thus, the solder bump is badly bonded to the pad. This poor bonding state not only precludes effective heat emission, but also increases contact resistance between the pad and the bump, thus resulting in an elevation of driving voltage.